Microstructure with movable mass

ABSTRACT

A microstructure includes a mass; a base member in which the mass is movably contained. The mass includes a surface, which is exposed out of the base member, and a stopper wire, which is arranged above the surface of the mass so as to inhibit over move of the mass.

TECHNICAL FIELD

The present invention relates generally to a microstructure with amovable mass (or moving mass). The present invention particularlyrelates to an accelerometer, which detects physical movement of a mass.

BACKGROUND ART

There are a variety of types of sensors using a movable mass(movingmass). For example, there is an inertial sensor such as an accelerometerand an angular accelerometer (vibration gyroscope).

An accelerometer, which detects an acceleration of a vehicle, generallyuses a piezoresistive effect. According to such a sensor, for example, abox shape of seismic mass (i.e. moving mass) is contained in a cavity ofa silicon base member (fixed frame). The movable mass is suspended bybeams on which piezoresistors are formed, so that a stress is applied tothe piezoresistors in response to movement of the mass. The variation ofstress applied to the piezoresistors is detected as a variation ofresistance. Such technology can be used for cruise control of vehicles.

The above-described mass is required to move freely, however, if themass over moves, the sensor may be broken or damaged. According to aninvention shown in Japanese Patent Publication Kokoku H5-71448, a glassstopper is used for inhibiting over-move of the mass.

However, because the glass stopper is arranged above the mass, thethickness of the sensor is increased, and the fabrication process ismore complicated. Thus fabrication costs are increased. Further, stressgenerated between the glass and silicon member may negatively affect thecharacteristics of the sensor.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide amicrostructure, in which over-move of a mass can be inhibited withoutsignificant increase of thickness. Another object of the presentinvention is to provide a microstructure, in which an over-moveinhibiting mechanism is realized without without negatively affectingthe characteristics of the microstructure and with increasing thecomplexity or expense of its fabrication.

Additional objects, advantages and novel features of the presentinvention will be set forth in part in the description that follows, andin part will become apparent to those skilled in the art uponexamination of the following or may be learned by practice of theinvention. The objects and advantages of the invention may be realizedand attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

According to a first aspect of the present invention, a microstructureincludes a mass, and a base member in which the mass is movablycontained. The mass includes a surface, which is exposed out of the basemember, and a stopper wire, which is arranged above the surface of themass so as to inhibit over move of the mass. As compared to theconventional technology using a glass stopper, the microstructure of thepresent invention can be fabricated to have a reduced thickness. Whenstopper wires are fixed, no stress is applied to the base member and themass. As a result, no negative affects are imparted to thecharacteristics of the microstructure, which can be an accelerometer.Further, the stopper wires can be fixed in a wire-bonding process, sothat the fabrication process can be uncomplicated, thereby allowing thecost of fabrication to be minimized. Especially, when the microstructureis an accelerometer, the stopper wires can be formed at the same timewhen electrode pads of a sensor chip and lead pads of a package are wirebonded for electrical connection.

According to a second aspect of the present invention, a packagestructure containing the above-described microstructure includes atleast one lead pad used for electrical connection with themicrostructure. At least one end of the stopper wire is connected to thelead pad. The stopper wires can be arranged freely and be changed easilyin design. When one end of the stopper wire is connected to an electrodepad of the base member and the other end is connected to a lead pad ofthe package, the stopper wire can be used not only for mechanicalprotection but also for electrical connection.

According to a third aspect of the present invention, the stopper wireincludes ends to be fixed at positions which are relatively lower inlevel than the surface of the mass. It is difficult to arrange a bondingwire having a height or clearance. If a clearance of a bonding wire isless than 100 μm, the clearance would be made irregularly. According tothe present invention, a distance or clearance between an upper surfaceof a mass and a stopper wire can be decreased without increasing adistance in vertical direction between ends and top of the wire. As aresult, the thickness of a microstructure can be reduced withoutincreasing irregularity of clearance or the height of the stopper wires.

According to a fourth aspect of the present invention, an accelerometerincludes a mass, and a silicon base member in which the mass is movablycontained, wherein the mass comprises a surface which is exposed out ofthe base member, a stopper wire which is arranged above the surface ofthe mass so as to inhibit over move of the mass; and a package whichcontains the base member and the mass.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating an accelerometer according toa first preferred embodiment of the present invention.

FIG. 2 is a plane view showing the accelerometer shown in FIG. 1, inwhich small elements like metal interconnection are omitted.

FIG. 3 is a cross-sectional view taken on line I—I in FIG. 1.

FIG. 4 is a cross-sectional view showing a part of the accelerometershown in FIG. 1.

FIG. 5 is a plane view showing another arrangement of stopper wires usedfor an accelerometer according to the first preferred embodiment.

FIG. 6 is a plane view showing another arrangement of stopper wires usedfor an accelerometer according to the first preferred embodiment.

FIG. 7 is a cross-sectional view showing a package structure containingand an accelerometer according to a second preferred embodiment of thepresent invention.

FIG. 8 is a plane view showing an arrangement of stopper wires used foran accelerometer according to the second preferred embodiment.

FIGS. 9-12 are plane views showing other arrangements of stopper wiresused for the second preferred embodiment, shown in FIG. 7.

FIG. 13 is a cross-sectional view showing a package structure containingand an accelerometer according to a third preferred embodiment of thepresent invention.

FIG. 14 is a plane view showing an arrangement of stopper wires used foran accelerometer according to the third preferred embodiment.

FIG. 15 is a plane view showing another arrangement of stopper wiresused for an accelerometer according to the third preferred embodiment.

FIG. 16 is a cross-sectional view showing an accelerometer according toa fourth preferred embodiment of the present invention.

FIG. 17 is a perspective view illustrating an accelerometer according toa fifth preferred embodiment of the present invention.

FIG. 18 is a plane view showing an arrangement of stopper wires used foran accelerometer according to the fifth preferred embodiment.

FIG. 19 is a cross-sectional view illustrating an accelerometeraccording to a sixth preferred embodiment of the present invention.

FIGS. 20A-20D are cross-sectional views showing one example offabricating steps of the accelerometer according to the sixth preferredembodiment, shown in FIG. 19.

FIGS. 21A-21G are cross-sectional views showing another example offabricating steps of the accelerometer according to the sixth preferredembodiment, shown in FIG. 19.

FIGS. 22A-22F are cross-sectional views showing another example offabrication steps of the accelerometer according to the sixth preferredembodiment, shown in FIG. 19.

FIG. 23 is a cross-sectional view illustrating an accelerometeraccording to a seventh preferred embodiment of the present invention.

FIGS. 24A-24E are cross-sectional views showing an example offabrication steps of the accelerometer according to the seventhpreferred embodiment, shown in FIG. 23.

FIG. 25 is a perspective view illustrating an accelerometer according toan eighth preferred embodiment of the present invention.

FIG. 26 is a cross-sectional view illustrating an accelerometeraccording to the eighth preferred embodiment, shown in FIG. 25.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,in which is shown, by way of illustration, specific preferredembodiments in which the invention may be practiced. These preferredembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. However, it is to be understoodthat other preferred embodiments may be utilized, and that logical,mechanical and electrical changes may be made without departing from thespirit and scope of the present invention. The following detaileddescription is therefore not to be taken in a limiting sense, and thescope of the present invention is defined only by the appended claims.

Now the present invention is described. The present invention isapplicable to a variety of types of inertial sensors, such as anaccelerometer, and an angular accelerometer (vibration gyroscope). Thepresent invention is also applicable to any kind of microstructure(MEMS) having a movable member, such as an actuator.

FIGS. 1 and 2 show an accelerometer 10 according to a first preferredembodiment of the present invention. In FIG. 2, small parts, such asmetal interconnection, are not shown for better understanding of overallof the accelerometer 10. FIG. 3 shows an inside structure of theaccelerometer 10. The accelerometer 10 includes a silicon base member12, and a movable mass 14, which is contained in a cavity of the siliconbase member 12. The mass 14 is provided so as to be able to moveup-and-down and right-and-left in three-dimensions. The silicon basemember 12 has at its center a square shaped cavity, in which the mass 14is contained. The movable mass 14 is shaped to be a cloverleaf havingfour square regions, which are connected at the center thereof, in orderto increase inertia force. An upper surface of the movable mass 14 andan upper surface of the base member 12 are arranged in the same level.

The accelerometer 10 further includes four beams 16, which connect themass 14 and base member 12, and eight piezoresistance elements 18. Thepiezoresistance elements 18 are arranged at the boundaries between themass 14 and the beams 16, between the base member 12 and the beams 16.Each of the beams 16 is arranged at a gap formed between two adjacentsquare parts of the mass 14. The silicon base member 12 is provided atthe upper surface with electrode pads, which are connected to thepiezoresistance elements 18 with a metal interconnection (not shown).

Electrode pads 20, which are not connected to the piezoresistanceelements 18, are connected to ends of stopper wires 22. Four of thewires 22 are arranged above all the corners of the movable mass 14. Eachwire 22 is arranged to extend across a corner of the movable mass 14.The ends of the wire 22 are fixed to the electrode pads 20 by aconventional wire-bonding process.

As shown in FIG. 3, the silicon base member 12 is fixed on to a die bondsurface 24. Downward over move of the movable mass 14 is inhibited bythe die bond surface 24. Horizontal over-move of the mass 14 isinhibited by the inner walls of the silicon base member 12. Upwardover-move of the mass 14 is inhibited by the wires 22. The clearancebetween the mass 14 and the wires 22 can be adjusted by controlling thewire bonding device. “Over-move,” means movement that causes theaccelerometer 10 to not work. For example, if the mass 14 over moves,the accelerometer 10 would be broken or would output a signal at a levelover its maximum rated level.

FIG. 4 shows a structure around an end of the wire 22 connected to thepad 20. According to the present invention, connecting regions betweenthe wire 22 and the electrode pad 20 can be covered with a resin 28. Asa result, the reliability of connection between the wire 22 and the pad20 is increased.

To fabricate the above-described accelerometer 10, a SOI wafer is formedfrom a silicon layer (Si), a buried oxide layer (SiO2) and a Sisubstrate. A bridge circuit is formed on the SOI wafer using asemiconductor technology to form piezoresistance elements 18, a metalcircuit pattern and electrode pads 20. After that, the surface iscovered with a passivating film, such as SiN layer, except the electrodepads 20. Next, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRIE process carried out from the Si substrate. Next, the movable mass 14is released from the substrate in an etching process to the buried oxidelayer. After that, the substrate is cut to form individual sensor chipsin a dicing process. Next, the sensor chip is bonded in a package, then,the electrode pads 20 of the sensor chip 10 and lead pads of the packageare wire bonded for electrical connection. At the same time, the wires22 are formed over the sensor chip 10.

FIGS. 5 and 6 show an example of the arrangement of the stopper wire(22). According to FIG. 5, two of wires 122 are arranged in parallel tothe sides of the silicon base member 12. These two wires 122 have thesame length. According to FIG. 6, three of wires 222 a and 222 b arearranged in parallel to a diagonal line of the silicon base member 12. Alonger wire 222 a extends along a diagonal line of the silicon basemember 12. Shorter wires 222 b are extend to across over coiners of themovable mass 14 in the same manner as shown in FIG. 1. In FIG. 6, theshorter wires 222 b can be omitted, because the longer wire 222 a wouldbe able to inhibit overmove of the mass 14 and keep the mass level.

FIG. 7 is a cross-sectional view showing a package structure containingand an accelerometer 10 according to a second preferred embodiment ofthe present invention. In FIG. 7, the elements that are the same orcorrespond to elements in the first preferred embodiment are representedby the same reference number, and the same description is not repeatedhere in this embodiment. According to the second preferred embodiment,both ends of stopper wire 22 are fixed onto lead pads 32 of a package30. As compared to the first preferred embodiment, the wire 22 can bearranged with a high degree of freedom.

FIGS. 8-12 are plane views showing example arrangements of stopper wires22 used for the second preferred embodiment, shown in FIG. 7. Accordingto the example shown in FIG. 8, two of wires 22 are arranged to extendin parallel to sides of the accelerometer 10. According to the examplesshown in FIG. 9, four of wires 22 are arranged to extend across thecorners of the accelerometer 10, in which none of the wires are crossedto each other. According to the example shown in FIG. 10, four of wires22 are arranged to form a shape of “#” above the accelerometer 10.According to the example shown in FIG. 11, two shorter wires 22 and twolonger wires 22 are arranged to extend in parallel to a diagonal line ofthe accelerometer 10. According to the example shown in FIG. 12, four ofwires 22 are arranged to form a diamond shape, in which each wire 22extend across a corner of the accelerometer 10.

FIG. 13 is a cross-sectional view showing a package structure containingand an accelerometer according to a third preferred embodiment of thepresent invention. In FIG. 13, the same or corresponding elements tothose in the first and second preferred embodiment, are represented bythe same reference number, and the same description is not repeated herein this embodiment.

According to the third preferred embodiment, one end of a stopper wire22 is bonded onto a lead pad 32 of a package 30, and the other end isbonded on to an electrode pad 20 of an accelerometer 10. The stopperwires 22 can be used for electrical connection between the package 30and the sensor 10. The lead pad 32 and electrode pad 20 can be formedfor exclusive use for the stopper wires 22, so that the stopper wires 22can be electrically isolated from the sensor 10, As a result, thestopper wires 22 can be arranged without taking into consideration of awiring design of the sensor 10.

FIGS. 14 and 15 are plane views showing example arrangements of thestopper wires 22 used for the accelerometer 10 according to the thirdpreferred embodiment. In a practical example shown in FIG. 14, four ofwires 22 are arranged to form a shape of “#” above the accelerometer 10.In a practical example shown in FIG. 15, four of wires 22 are arrangedto form a diamond shape, in which each wire 22 extend across a corner ofthe accelerometer 10.

FIG. 16 is a cross-sectional view showing an accelerometer according toa fourth preferred embodiment of the present invention. In FIG. 16, thesame or corresponding elements to those in the first to third preferredembodiments are represented by the same reference number, and theirdescription is not repeated here in this embodiment. A feature of thisembodiment is that stopper wires 22 are provided to have the sameelectrical potential level with an SOI wafer 50, so that the wires 22 donot operate as an antenna, which affects negatively to characteristicsof the accelerometer 10.

In fabrication of the above-described accelerometer 10, a SOI wafer isformed from a silicon layer (Si), a buried oxide layer (SiO2) and a Sisubstrate. A bridge circuit is formed on the silicon layer in an ionimplantation or thermal diffusion process to form piezoresistanceelements 18. After that, an insulating layer 52 is formed in a thermaloxidation process. Next, contact holes are formed so as that anelectrode pad 20, which is connected to the wire 22, becomes the sameelectrical potential level as the silicon layer (Si). After that, ametal circuit pattern and electrode pads 20 are formfeed, and thesurface is covered with a passivating film 54, such as SIN layer, exceptthe electrode pads 20.

Next, beams 16 are formed by a Si Deep RIE (Reactive Ion Etching)process. After that, the movable mass 14 is formed in a Si Deep RIEprocess carried out from the Si substrate. Next, the movable mass 14 isreleased from the substrate in an etching process to the buried oxidelayer. After that, the substrate is cut to form individual sensor chipsin a dicing process. Next, the sensor chip 10 is bonded in a package.Then, the electrode pads 20 of the sensor chip 10 and lead pads of thepackage are wire bonded for electrical connection. At the same time, thewires 22 are formed over the sensor chip 10.

FIGS. 17 and 18 show an accelerometer 10 according to a fifth preferredembodiment of the present invention. In FIG. 18, small parts, such asmetal interconnection, are not shown for better understanding of overallof the accelerometer 10. FIG. 19 shows an inside structure of theaccelerometer 10. The accelerometer 10 includes a silicon base member12, and a movable mass 14, which is contained in a cavity of the siliconbase member 12. The mass 14 is provided so as to be able to moveup-and-down and right-and-left in three-dimensions. The silicon basemember 12 is provide at its center with a square shape cavity, in whichthe mass 14 is contained. The movable mass 14 is shaped to be acloverleaf having four square regions, which are connected at the centerthereof, in order to increase inertia force.

Projected member 14 a is formed on each of the four square regions ofthe mass 14 so that an upper surface of the projected member 14 a is“Δh” higher than an upper surface of the silicon base member 12.

The accelerometer 10 further includes four beams 16, which connect themass 14, base member 12, and eight of piezoresistance elements 18. Thepiezoresistance elements 18 are arranged at the boundaries between themass 14 and the beams 16, and between the base member 12 and the beams16. Each of the beams 16 is arranged at a gap formed between twoadjacent square parts of the mass 14. The silicon base member 12 isprovided at the upper surface with electrode pads, which are connectedto the piezoresistance elements 18 with a metal interconnection (notshown).

Electrode pads 20, which are not connected to the piezoresistanceelements 18, are connected to ends of stopper wires 22. Four of thewires 22 are arranged above all the corners of the movable mass 14. Eachwire 22 is arranged to extend across a corner of the movable mass 14.The ends of the wire 22 are fixed to the electrode pads 20 by aconventional wire-bonding process.

As shown in FIG. 19, the silicon base member 12 is fixed on to a diebond surface 24. Downward over move of the movable mass 14 is inhibitedby the die bond surface 24. Horizontal over move of the mass 14 isinhibited by inner walls of the silicon base member 12. Upward over-moveof the mass 14 is inhibited by the wires 22. The clearance between themass 14 and the wires 22 can be adjusted by controlling the wire bondingdevice.

A distance “H” between the upper surface of the projected member 14 aand the stopper wire 22 can be adjusted by a wire-bonding device (notshown) and the height difference “Δh”. According to this embodiment, adistance “H” can be decreased without increasing a distance “H+Δh”. As aresult, the thickness of the accelerometer 10 can be reduced withoutincreasing irregularity of clearance “H” of stopper wires 22. Even if adistance “H” is determined about 80 μm, irregularity of clearance “H” ofstopper wires 22 would not increase remarkably.

FIGS. 20A-20D are cross-sectional views showing one example of thefabricating steps of the accelerometer according to the sixth preferredembodiment, shown in FIG. 19. In fabricating the above-describedaccelerometer 10, a SOI wafer 31 was formed from a silicon layer (Si), aburied oxide layer (SiO2) and a Si substrate. A bridge circuit wasformed on the silicon layer in a semiconductor process to formpiezoresistance elements 18, a metal circuit pattern and electrode pads20. A sensor circuit 33 is formed, as shown in FIG. 20A.

Next, a photosensitive polyimide or resist is formed on to the sensorcircuit 33 using a spin-coating process, and is exposed, developed andbaked to form a projected member 34 (14 a), as shown in FIG. 20B.

Subsequently, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRIE process carried out from the Si substrate 31. Next, the movable mass14 is released from the substrate 31 in an etching process to the buriedoxide layer, as shown in FIG. 20C. After that, the substrate 31 is cutto form individual sensor chips in a dicing process. Next, the sensorchip is bonded in a package, and the electrode pads 20 of the sensorchip 10 and the lead pads of the package are wire bonded for electricalconnection. At the same time, the wires 22 are formed over the sensorchip 10.

FIGS. 21A-21G are cross-sectional views showing another example offabricating steps of the accelerometer 10 according to the sixthpreferred embodiment, shown in FIG. 19. In fabrication of the abovedescribed accelerometer 10, a SOI wafer 31 is formed from a siliconlayer (Si), a buried oxide layer (SiO2) and a Si substrate. A bridgecircuit is formed on the silicon layer in a semiconductor process toform piezoresistance elements 18, a metal circuit pattern and electrodepads 20. A sensor circuit 33 is formed, as shown in FIG. 21A.

Next, seed layer 40 is formed over the SOI wafer 31 in a sputteringprocess, as shown in FIG. 21B. The seed layer 40 may be of Ni, Cu, Au,Pd, Ag, Sn, Co or the like. After that, photosensitive polyimide orresist is formed on to the seed layer 40 in a spin-coating process, andis exposed, developed and baked to form a resist pattern 42, as shown inFIG. 21C. Next, a plating layer 40 a is formed from the seed layer 40,as shown in FIG. 21D. After that, the resist pattern 42 is removed, andthe seed layer 40 is removed in an ion-milling process, wet etchingprocess or RIE (Reactive Ion Etching) process to form a projected member44, as shown in FIG. 21E.

Subsequently, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRJE process carried out from the Si substrate 31. Next, the movable mass14 is released from the substrate 31 in an etching process to the buriedoxide layer, as shown in FIG. 21F. After that, the substrate 31 is cutto form individual sensor chips in a dicing process. Next, the sensorchip is bonded in a package, then, the electrode pads 20 of the sensorchip 10 and lead pads of the package are wire bonded for electricalconnection. At the same time, the wires 22 are formed over the sensorchip 10, as shown in FIG. 21G.

FIGS. 22A-22F are cross-sectional views showing another example of thefabrication steps used to manufacture the the accelerometer 10 accordingto the sixth preferred embodiment, shown in FIG. 19. In fabrication ofthe above described accelerometer 10, a SOI wafer 31 was formed from asilicon layer (Si), a buried oxide layer (SiO2) and a Si substrate.Next, a polysilicon layer 48 was formed on the SOI wafer 31, as shown inFIG. 22A. Subsequently, a photosensitive polyimide or resist was formedon to the polysilicon layer 48 in a spin-coating process, and wasexposed, developed and baked to form a resist pattern 51, as shown inFIG. 22B.

Next, the polysilicon layer 48 is etched in a RIE process to form aprojected member 53, as shown in FIG. 22C. Subsequently, a bridgecircuit is formed in a semiconductor process to form piezoresistanceelements 18, a metal circuit pattern and electrode pads 20. A sensorcircuit 33 is formed over the projected member 53, as shown in FIG. 22D.After that, the surface is covered with a passivating film, such as SiNlayer, except the electrode pads 20.

Subsequently, beams 16 are formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 is formed in a Si DeepRIE process carried out from the Si substrate 31. Next, the movable mass14 is released from the substrate 31 in an etching process to the buriedoxide layer, as shown in FIG. 22E. After that, the substrate 31 is cutto form individual sensor chips in a dicing process. Next, the sensorchip is bonded in a package, then, the electrode pads 20 of the sensorchip 10 and lead pads of the package are wire bonded for electricalconnection. At the same time, the wires 22 are formed over the sensorchip 10, as shown in FIG. 22F.

FIG. 23 is a cross-sectional view illustrating an accelerometeraccording to a seventh preferred embodiment of the present invention.FIGS. 24A-24E are cross-sectional views showing an example of thefabrication steps used to manufacture the accelerometer according to theseventh preferred embodiment, shown in FIG. 23. In FIGS. 23 and 24A-24E,the same or corresponding elements to those in the above-described sixthpreferred embodiment are represented by the same reference numeralshere, and their description is not repeated here in this embodiment. Thedifference between this embodiment and the six preferred embodiment isthat structure of a movable mass 14 and a silicon base member 53. Inthis embodiment, the mass 14 is not provided with any projected member(14 a), but an upper surface of the silicon base member 58 is cut out inorder to form a level difference “Δh” between the upper surface of thebase member 58 and an upper surface of the mass 14.

To fabricate the above-described accelerometer, a SOI wafer 31 wasformed from a silicon layer (Si), a buried oxide layer (SiO2) and a Sisubstrate. Next, a resist was formed on to the SOI wafer 31 in aspin-coating process, and was exposed, developed and baked to form aresist pattern 60, as shown in FIG. 24A. After that, the SOI wafer 31was etched in a RIE process to form recess portions 31a, which are usedas electrode pads, as shown in FIG. 24E.

Next, a bridge circuit was formed in a semiconductor process to formpiezoresistance elements 18, a metal circuit pattern and electrode pads20. A sensor circuit 33 is formed on the SOI wafer 31, as shown in FIG.24C. After that, the surface of the substrate 31 is covered with apassivating film, such as SiN layer, except the electrode pads 31 a.

Subsequently, beams 16 were formed in a Si Deep RIE (Reactive IonEtching) process. After that, the movable mass 14 was formed in a SiDeep RIE process carried out from the Si substrate 31. Next, the movablemass 14 was released from the substrate 31 in an etching process to theburied oxide layer, as shown in FIG. 24D. After that, the substrate 31was cut to form individual sensor chips in a dicing process. Next, thesensor chip was bonded in a package, then, the electrode pads 31 a ofthe sensor chip 10 and lead pads of the package were wire bonded forelectrical connection. At the same time, the wires 22 were formed overthe sensor chip 10, as shown in FIG. 24E.

FIG. 25 is a perspective view illustrating an accelerometer 74 accordingto an eighth preferred embodiment of the present invention. FIG. 26 is across-sectional view illustrating the accelerometer 74, shown in FIG.25. In FIGS. 25 and 26, the same or corresponding elements to those inthe above-described embodiments are represented by the same referencenumerals, and the same description is not repeated here in thisembodiment.

The accelerometer 74 includes a silicon base member 12, and a movablemass 14, which is contained in a cavity of the silicon base member 12.The mass 14 is provided so as to be able to move up-and-down andright-and-left in three-dimensions. The silicon base member 12 isprovided at its center with a square shape cavity, in which the mass 14is contained. The movable mass 14 is shaped to be a cloverleaf havingfour square regions, which are connected at the center thereof, in orderto increase inertia force. An upper surface of the movable mass 14 andan upper surface of the base member 12 are arranged in the same level.

The accelerometer 74 further includes four beams 16, which connect themass 14 and base member 12, and eight piezoresistance elements 18. Thepiezoresistance elements 18 are arranged at the boundaries between themass 14 and the beams 16, and between the base member 12 and the beams16. Each of the beams 16 is arranged at a gap formed between twoadjacent square parts of the mass 14. The silicon base member 12 isprovided at the upper surface with electrode pads 20, which areconnected to the piezoresistance elements 18 with a metalinterconnection (not shown).

The accelerometer 74 is contained in a package 70, shown in FIG. 26. Thepackage 70 includes recess portions 75 and lead pads 74, which areformed on the recess portions 75. The recess portions 75 and lead pads76 are designed so that an upper surface of the lead pad 76 is “Δh”lower than an upper surface of the accelerometer (sensor) 74. Two leadpads, arranged at the opposite sides of the accelerometer 74, areconnected by a stopper wire 22. According to the eight preferredembodiment, the same advantages can be obtained as the above describedsixth and seventh preferred embodiments.

According to the sixth to eighth preferred embodiments, the stopper wire22 includes ends to be fixed at positions that are relatively lower inlevel than the surface of the mass 14. A distance “H” between an uppersurface of the mass 14 and the stopper wire 22 can be decreased withoutincreasing a distance (H+Δh) in vertical direction between ends and topof the wire 22. As a result, the thickness of a microstructure can bereduced without increasing irregularity of clearance or heights ofstopper wires 22.

1. A microstructure having a movable mass, comprising: a mass; a basemember in which the mass is movably contained, wherein the masscomprises a surface which is exposed out of the base member, and astopper wire which is arranged above the surface of the mass so as toinhibit over move of the mass, wherein the stopper wire comprises endsto be fixed at positions which are relatively lower in level than thesurface of the mass.
 2. A microstructure according to claim 1, wherein aprojecting portion is formed on the surface of the mass.
 3. Amicrostructure according to claim 2, wherein the projecting portion is aresin layer formed on the surface of the mass.
 4. A microstructureaccording to claim 2, wherein the projecting portion is a plating layerformed on the surface of the mass.
 5. A microstructure according toclaim 2, wherein the projecting portion is a polysilicon layer formed onthe surface of the mass.
 6. A microstructure according to claim 1,wherein the base member is provided with wire-fixing regions which areconnected with the ends of the stopper wire, the wire-fixing regionsbeing formed to have surfaces lower in level than the surface of themass.
 7. A microstructure according to claim 6, wherein the wire-fixingregions are formed to shape the base member to be stepped structures. 8.An accelerometer which provides an output signal in accordance with themovement of a mass, comprising: a mass; a silicon base member in whichthe mass is movably contained, wherein the mass comprises a surfacewhich is exposed out of the base member; a stopper wire which isarranged above the surface of the mass so as to inhibit over move of themass; and a package which contains the base member with the mass,wherein the package comprises at least one lead pad used for electricalconnection with the microstructure, and at least one end of the stopperwire is connected to the lead pad.
 9. An accelerometer according toclaim 8, wherein both ends of the stopper wire is connected to the leadpads.
 10. An accelerometer according to claim 8, wherein one end of thestopper wire is connected to the lead pads, and the other end isconnected to an electrode pad of the base member.
 11. An accelerometeraccording to claim 8, wherein the stopper wire comprises ends to befixed at positions which are relatively lower in level than the surfaceof the mass.
 12. An accelerometer which provides an output signal inaccordance with the movement of a mass, comprising: a mass; a siliconbase member in which the mass is movably contained, wherein the masscomprises a surface which is exposed out of the base member; a stopperwire which is arranged above the surface of the mass so as to inhibitover move of the mass; and a package which contains the base member withthe mass, wherein the stopper wire comprises ends to be fixed atposition which are relatively lower in level than the surface of themass.
 13. An accelerometer according to claim 12, wherein the packagecomprises at least one lead pad used for electrical connection with themicrostructure, wherein at least one end of the stopper wire isconnected to the lead pad.
 14. An accelerometer according to claim 12,wherein both ends of the stopper wire are connected to the pads.
 15. Anaccelerometer according to claim 12, wherein one end of the stopper wireis connected to the lead pad, and the other end is connected to anelectrode pad of the base member.